1. Field of the Invention
This invention relates to composite materials. In particular, it relates to an aqueous process to prepare carbon fiber/polymer powder prepregs using an aqueous slurry technique.
2. Description of the Related Art
High temperature polymeric thermoplastic resins are attractive to the aerospace industry because of their excellent mechanical properties which are retained over a wide temperature range. The beneficial properties of such polymers as polyimides include strength, toughness and adhesion which makes them prime candidates as matrix resins in advanced carbon-fiber reinforced composites. However, preparing composites of these materials has been difficult due to their lack of melt flow and insolubility in standard solvents. Successful composite manufacture must be preceded by a pre-impregnation step in which the resin is intimately impregnated into the carbon fibers so as to require minimal melt flow to achieve composite consolidation.
Various techniques have been developed to combine thermoplastic resins and reinforcing fibers to form prepreg materials. In these techniques, polymer is deposited on the fiber from the melt state, from solution, or in powder form from a slurry or a fluidized bed. Many thermoplastic resins, such as polyimides, have poor melt flow and are insoluble in most solvents but may be prepared in powder form.
Dyksterhouse et al (U.S. Pat. No. 4,894,105) formed composites by dispersing polymer particles in an aqueous medium containing an effective amount of a dissolved polymeric binding agent. This binding agent is polyacrylic acid. The viscosity of the medium must be at least 50,000 cps to form a gelled impregnation bath wherein the polymer particles are uniformly suspended. This viscosity requirement is significantly higher than the present invention, where the viscosity is only 300 cps.
Pike (U.S. Pat. No. 4,480,088) teaches a process to prepare water soluble polyimide resin systems. This process involves the formation of the poly(amic acid) salt which may be converted to the polyimide by heating. There is no mention of the use of the poly(amic acid) salt as a surfactant for polymeric powders in an aqueous system.
Neither of the two previously mentioned inventions teaches the process for preparing a consolidated composite laminate using a poly(amic acid) salt as a surfactant. By the present invention, there is no need to increase the viscosity of the aqueous medium as was required in the related art. Also, the use of the poly(amic acid) as a surfactant allows for better compatibility throughout the final composite.